1. Field of the Invention
The present invention relates to a resistance element used for an electron gun of a cathode ray tube that supplies an anode voltage divided into a deflection voltage, an electrode voltage, etc. and a cathode ray tube having the same.
2. Description of the Related Art
For example, in a cathode ray tube (CRT) of the Trinitron system (trade mark of SONY CORPORATION), an electron beam is made to converge by supplying a voltage 4 to 8 percent lower than the anode voltage (convergence voltage) to a static deflection plate. One of the methods used for supplying this convergence voltage to a deflection plate is the use of a resistance element known as an inner breeder resistor (IBR).
In a cathode ray tube having such an IBR, as shown by the sectional view of the neck portion of the cathode ray tube in FIG. 1, a high anode voltage is supplied from the outside of the cathode ray tube 10 from an anode button 11 through the interior carbon 12 to the electron gun 20 and IBR. This high anode voltage is divided into the convergence voltage by the IBR and is then supplied to the deflection plate 26.
This IBR and electron gun are represented in FIGS. 2A and 2B. The electron gun 20 is of a unipotential type and has a first grid G.sub.1, second grid G.sub.2, third grid G.sub.3, fourth grid G.sub.4, fifth grid G.sub.5, and deflection plate 26 arranged coaxially in order from the rear side of the cathode ray tube 10 to the screen side. For example, the first grid G.sub.1 is supplied with 0V, the second grid G.sub.2 with 300V, the third grid G.sub.3 with 27 kV, the fourth grid G.sub.4 with 8 kV, and the fifth grid G.sub.5 with 27 kV. The deflection plate 26 is supplied with the high voltage from the fifth grid G.sub.5 through an electrode A divided by the IBR and passed through the electrode C.
The IBR, as shown in FIG. 2B, is flat and is laid near the grids G.sub.1 to G.sub.5 stretching between the fifth grid G.sub.5 and the first grid G.sub.1. On the surface of the flat substrate 31 are formed resistors 32 between the electrode A and electrode C and between the electrode C and electrode B. The electrode A is connected to the fifth grid G.sub.5, the electrode C is connected to the deflection plate 26, and the electrode B is connected to the outside power supply of 300 to 1000V through a stem 27.
A plan view of the IBR is given in FIG. 3A and a side view in FIG. 3B. The IBR is comprised of a plate 96 percent alumina high insulation substrate 31 on which are formed electrodes A, B, and C using a ruthenium oxide family low resistance paste. Between the electrode A and electrode C, and between the electrode C and electrode B are formed resistors 32 in a wavy manner by coating and baking ruthenium oxide family paste. The resistors 32 are protected by being covered with high voltage resistance, high insulation glass frit (called an "overcoat" in some instances) (B.sub.2 O.sub.3 --SiO.sub.2 --PbO family) 33a and 33b. Further, the glass frit 33c is formed on the opposite surface of the surface on which the resistors are formed. The substrate 31 is covered by baking the resistors.
An enlarged view of the area near the electrode B is shown in FIG. 4. The overcoat layers 33a, 33b, and 33c are coated via silk printing to form thick layers. The overcoat layers tend to suffer from pinholes PH and bubbles BB. When the insulation is defective as a result of these, the resistors 32 are damaged. To provide enough insulation, the resistors 32 have formed over them two additional layers: the first overcoat film 33a and the second overcoat film 33b. The overall thickness of the combined films is about 0.5 mm. The overcoat film 33c formed on the surface opposite the surface where the resistors 32 are formed is for preventing the release of gas from the substrate 31, and has a thickness of, for example, about 20 .mu.m. Further, an overcoat film 33d is formed on the ends of electrode B and the electrode C.
A high voltage of about 27 kV is supplied to the electrode A of the IBR. There is a large potential difference with the electrode B where the voltage is 300 to 1000V. The surface of the high insulation overcoat film 33b is charged up and the voltage gradually increases toward the electrode B. Finally, there is a discharge with the electrode B or with the first grid G.sub.1, second grid G.sub.2, and fourth grid G.sub.4 of the electron gun 20 and further with the resistors 32. The state of this discharge Is explained with reference to FIG. 14.
In the IBR shown in FIG. 5A, the resistor 32 has a higher wave density and therefore a higher resistance at the electrode B side. The IBR is arranged near the electron gun shown in FIG. 5B. The density of the waves in the resistor 32 is changed, as shown in FIG. 6, to ease the gradient of potential of the resistor at the portion of the resistor 32 with the low wave density (and therefore lower resistance), as shown by (3), and to reduce the potential difference with the third grid G.sub.3 (where anode potential is applied). If the cathode ray tube is used and a current flows to the resistor 32 of the IBR, the heat generated at the high resistance part will be great. Therefore, at the part with a low resistance heat of about 80.degree. C. will be generated, but at the part with a high resistance, the temperature has been observed to reach 150.degree. C.
Since the overcoat films 33a and 33b covering the resistors 32 have high insulating properties, the surface of the overcoat film 33b is charged up to a high potential. When the electron gun starts to be used, as shown by the line (1) in FIG. 6, the potential falls substantially linearly from the electrode A to the electrode B, but when a given time elapses, as shown by the line (2), the voltage gradually rises toward the electrode B, then finally a discharge occurs with the electrode B or with the first grid G.sub.1, the second grid G.sub.2, and further the fourth grid G.sub.4 and further the resistors 32. When the discharge occurs, the charge is released and the potential returns to the line (1), but when a given time again elapses, the charge again rises and a similar discharge occurs in a repeating pattern. Due to this discharge, insulation breakdown of the overcoat films 33a and 33b occurs and the resistors 32 are damaged, whereupon the potential of the electrode C changes, the convergence voltage changes, and the color on the screen ends up becoming wrong.
The insulation of the IBR is therefore, very important. To ensure good insulation, the overcoat film on the resistors 32 is applied twice to form a thickness of 0.4 to 0.5 mm as a whole, and the finished IBRs are strictly inspected visually. This slower manufacturing process often results in poorer productivity.